CHICAGO (CBS) — It turns out Chicago’s favorite Tyrannosaurus rex was much heavier than anyone knew.

As WBBM Newsradio’s Kris Kridel reports, until now, calculating the weight of dinosaurs has been an unreliable discipline.

But in the latest study, scientists weighed actual skeletons, including that of Sue, the T. Rex at the Field Museum.

LISTEN: WBBM Newsradio’s Kris Kridel reports

“Previous methods for calculating mass relied on scale models, which can magnify even minor errors, or on extrapolations from living animals with very different body plans from dinosaurs,” Field Museum curator of dinosaurs Dr. Peter Makovicky said in a news release. “We overcame such problems by using the actual skeletons as a starting point for our study.”

It turned out Sue weighed about 9 tons, one third more than the 6 tons originally thought.

“We knew she was big but the 30 percent increase in her weight was unexpected,” Makovicky said in the release.

Led by Makovicky and Professor John R. Hutchinson of the Royal Veterinary College in London, the scientists also used 3-D laser scans to generate a fleshed-out digital model whose mass could be computed, the news release explained.

The project involved constructing digital mockups of body cross-sections, which were reconstructed across the length of Sue’s skeleton, using the relationship between soft tissues and skeletons of birds and crocodiles for guidance, the release explained. A digital skin was also developed, and a mass was calculated, following the subtraction of empty spaces such as the lungs and oral cavity.

The masses of several other dinosaur skeletons were also measured, and Sue came out the largest.

Based on the research, the scientists believe the T. rex grew at a rate of 3,950 pounds per year during their teenage period of growth, the release said.